1. Field of the Invention
The present invention relates to an electric motor and an actuator having the same.
2. Description of Related Art
An electric motor, which drives a drive subject to rotate the same, is known.
When an excessive braking torque is generated in the electric motor, for example, at the time of locking of the drive subject, a mechanical damage may possibly be applied to components of the drive subject by an excessive rotational torque generated by the electric motor.
The above disadvantage will be illustrated by using an example of a shift range change apparatus of a shift-by-wire (SBW) type.
An automatic transmission of a vehicle includes a shift range change mechanism (including a parking change mechanism) and is shifted manually by a driver of the vehicle. However, lately, a shift range change apparatus, which changes the shift range change mechanism by an SBW actuator having an electric motor, is prevailing in the market (see, for example, Japanese Unexamined Patent Publication No. 2006-191709 corresponding to US 2006/0138880A1).
In the electric motor, when the temperature gets high, the electric resistance becomes large. In such a case, the drive current of the electric motor drops, and the output torque of the electric motor becomes small. Also, at the low battery voltage, the drive current drops, so that the output torque of the electric motor becomes small. Accordingly, in the high temperature and low battery voltage state, the drive current of the electric motor becomes minimum (high temperature×low voltage=minimum drive current).
Here, it is demanded that the electric motor of the shift range change apparatus generates the required torque even under the worst condition of “high temperature×low voltage state=minimum drive current”.
However, when the required torque is ensured in the worst condition of “high temperature×low voltage=minimum drive current”, a large rotational torque would be produced in an opposite condition of “low temperature×high voltage=maximum drive current”.
Here, at the time of changing the shift range, the electric motor receives only the reaction force, which corresponds to the torque required for the changing of the shift range, from the drive subject (from the shift range change mechanism side). Thus, at the time of changing the shift range, the excessive rotational torque (excessive load) is not exerted on the rotor shaft of the electric motor independent of the temperature and the voltage.
However, when the drive subject is placed in the locked state under the condition of “low temperature×high voltage”, the electric motor tries to drive the locked drive subject, and thereby an excessive rotational torque (excessive load) is applied to the drive subject. Therefore, a mechanical damage may possibly be applied to the components of the rotation transmission system (including a speed reducer), which is from the electric motor to the drive subject, or the components of the drive subject.
Specifically, in a known shift range change apparatus, an abutment control operation is executed to learn a reference position of the motor (one rotational position in a movable range of the electric motor) by moving and hitting a movable member of the drive subject to one end of a rotatable range.
When this abutment control operation is executed under the condition of “low temperature×high voltage”, the excessive rotational torque of the electric motor may possibly be applied to the drive subject, and thereby the mechanical damage may possibly be applied to the components of the rotation transmission system from the electric motor to the drive subject or the components of the drive subject.